The Australian-born peeress, affectionately known as Kanga by her royal pal, died within weeks of Princess Diana from septicemia in 1997.

While Charles had once described Lady Tryon as “the only woman who really understands me”, by the mid-Seventies she had lost out to her old rival Camilla Parker Bowles when it came to the Prince’s affections.

“The documentary is going to be screened on Channel 4 and will shed fresh light on what was an eventful but ultimately sad story,” says an insider. “It’s also expected to highlight the rivalry between Kanga and Camilla, and how Camilla eventually ensured she was the woman in Charles’s life, despite being married herself by that stage.”

Kanga is said to have embarked on a relationship with Charles in 1973 – when his long-term mistress Camilla married Andrew Parker Bowles.

Kanga’s later years were overshadowed by poor health and addiction problems.

Her increasingly bizarre antics, said to have been caused by her medication, saw her regularly hit the headlines – notably when she was sectioned months before her death.